1. Field of the Invention
The present invention relates to a zoom lens and an optical apparatus having the same, and more particularly to a zoom lens of the rear-focus type having a high variable magnification ratio and a large aperture ratio, the number of constituent lens elements of which is relatively small, and which is suited for use with a video camera, a film camera, a broadcasting camera, or the like, and an optical apparatus having the same.
2. Description of Related Art
Heretofore, in zoom lenses for use with photographic cameras, video cameras or the like, there have been proposed a variety of zoom lenses adopting the so-called rear-focus type lens, in which focusing is performed by moving a lens unit disposed behind a first lens unit, which is disposed on the object side. This is because the zoom lens of the rear-focus type has such advantages that a small driving force sufficies for moving the focusing lens unit, which is relatively small and light, and rapid focusing can be attained, so that the zoom lens of the rear focus type matches an automatic focusing system.
Such zoom lenses of the rear focus type have been proposed, for example, in Japanese Laid-Open Patent Applications No. Sho 62-206516, No. Sho 62-24213, No. Sho 62-247316 and No. Hei 4-43311 (corresponding to U.S. Pat. No. 5,189,558). Each of these zoom lenses of the rear focus type comprises four lens units, i.e., in order from an object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, wherein the second lens unit is moved to effect a variation in magnification and the fourth lens unit is moved to compensate for the shift of an image plane due to the variation of magnification and to effect focusing.
Further, in Japanese Laid-Open Patent Application No. Sho 63-29718, there has been disclosed a zoom lens comprising, in order from an object side, a first lens unit of positive refractive power, a second lens unit composed of a negative lens, a negative lens and a positive lens, having a negative refractive power as a whole and arranged to be movable during a variation in magnification to mainly take charge of the variation of magnification, a third lens unit having a positive refractive power and including an aspheric surface, being followed by a relatively large air separation, and a fourth lens unit having a positive refractive power and arranged to move to compensate for the shift of an image plane due to the variation of magnification and to effect focusing.
In Japanese Laid-Open Patent Application No. Hei 5-72472 (corresponding to U.S. Patent No. 5,572,364), there has been disclosed a zoom lens unit of positive refractive power, a second lens unit of negative refractive power used for a variation in magnification, a fixed third lens unit of positive refractive power having a light condensing function, and a fourth lens of positive refractive power arranged to move along an optical axis so as to maintain the position of an image plane. In this zoom lens, the second lens unit is composed of a negative lens of meniscus form, a bi-concave lens and a positive lens, the third lens unit is composed of a single lens having one or more aspheric surfaces, and the fourth lens unit is composed of a lens having one or more aspheric surfaces.
In U.S. Patent No. 4,299,454, there has been disclosed a zoom lens comprising, in order from an object side, a positive first lens unit, a negative second lens unit and a positive rear lens unit, wherein a variation in magnification is effected by moving at least two lens units including the negative lens unit, and the negative second lens unit consists of, in order from the object side, first and second negative lenses and a positive doublet. In that U.S. Patent, there is disclosed no embodiment in which the second lens unit has an aspheric surface.
Further, in Japanese Laid-Open Patent Application No. Hei 8-292369 (corresponding to U.S. Pat. No. 5,940,221), there has been disclosed a zoom lens comprising four lens unit, i.e., in order from an object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power having an aspheric surface, a third lens unit of positive refractive power having an aspheric surface, and a fourth lens unit of positive refractive power having an aspheric surface, wherein the second lens unit is moved to effect a variation in magnification and the fourth lens unit is moved to compensate for the shift of an image plane due to the variation of magnification and to effect focusing.
Further, in the so-called four-unit zoom lens of rear focus type, there is a method of strengthening the refractive power of a second lens unit used for a variation in magnification, so as to reduce the amount of movement of the second lens unit along an optical axis required for securing a predetermined variable magnification ratio.
In the zoom lens adopting the above method, the separation between the first lens unit and the second lens unit, which serve as a magnification varying system, is shortened, and the distance from the stop to the first lens unit is shortened, so that the diameter of the front lens member can be reduced. Accodingly, because it becomes possible to make thin the thickness of the first lens unit, it is possible to minimize the entirety of the lens system.
Further, as one of methods of minimizing the third lens unit and the fourth lens unit, which serve as an image forming system, there is a method of shortening the real distance interval between the third lens unit and the fourth lens unit by constructing the third lens unit with a positive lens and a negative lens in order from an object side, i.e., making the third lens unit into a telephoto-lens type, and shifting the principal point position of the third lens unit to the object side. Zoom lenses adopting such a method have been proposed, for example, in Japanese Laid-Open Patent Application No. Hei 5-19165 (corresponding to U.S. Pat. No. 5,231,540), Japanese Laid-Open Patent Application No. Hei 5-297275 (corresponding to U.S. Pat. No. 5,396,367), Japanese Laid-Open Patent Application No. Hei 5-60973, Japanese Laid-Open Patent Application No. Hei 5-60974, Japanese Laid-Open Patent Application No. Hei 5-107473, Japanese Laid-Open Patent Application No. Hei 6-130297 (corresponding to U.S. Pat. No. 5,396,367), Japanese Laid-Open Patent Application No. Hei 8-304700, U.S. Pat. Nos. 5,189,558, and 5,396,367.
It is an object of the invention to improve the conventional zoom lens of the rear-focus type and provide a zoom lens that has a novel construction and a further-improved performance, and an optical apparatus having the zoom lens.
To attain the above object, in accordance with a first aspect of the invention, there is provided a zoom lens comprising, in order from an object side to an image side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, wherein a variation in magnification from a wide-angle end to a telephoto end is effected by moving the second lens unit toward the image side, and shifting of an image plane due to the variation of magnification is compensated for by moving the fourth lens unit, and focusing is effected by moving the fourth lens unit, a stop being disposed on the most object side of the third lens unit, the third lens unit comprising a positive lens (G3F) disposed on the image side of the stop and just behind the stop and having a convex lens surface facing the object side, at least one of lens surfaces of the positive lens (G3F) being an aspheric surface, and a negative lens (G3R) of meniscus form disposed on the most image side and having a concave surface facing the image side, the curvature of which is stronger than that of an opposite surface thereof, the fourth lens unit consisting of a cemented lens composed of a positive lens, both lens surfaces of which are convex and at least one lens, surface of which is an aspheric surface and a negative lens, and the following conditions being satisfied:
0.37 less than xcex24T less than 0.49
0.6 less than f3/f4 less than 1.15
where xcex24T is a magnification of the fourth lens unit obtained when focusing on an infinitely distant object at the telephoto end, and f3 and f4 are focal lengths of the third lens unit and the fourth lens unit, respectively.
In accordance with a second aspect of the invention, there is provided a zoom lens comprising, in order from an object side to an image side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, wherein a variation in magnification from a wide-angle end to a telephoto end is effected by moving the second lens unit toward the image side, shifting of an image plane due to the variation in magnification is compensated for by moving the fourth lens unit and focusing is effected by moving the fourth lens unit, a stop being disposed on the most object side of the third lens unit, the third lens unit comprising a positive lens (G3F) disposed on the image side of the stop and just behind the stop and having a convex lens surface facing the object side, at least one of lens surfaces of the positive lens (G3F) being an aspheric surface, and a negative lens (G3R) of meniscus form disposed on the most image side and having a concave surface facing the image side, the curvature of which is stronger than that of an opposite surface thereof, the fourth lens unit consisting of a cemented lens composed of a positive lens, both lens surfaces of which are convex and at least one lens surface of which is an aspheric surface and a negative lens, and the following condition being satisfied:
0.37 less than xcex24T less than 0.43
where xcex24T is a magnification of the fourth lens unit obtained when focusing on an infinitely distant object at the telephoto end.
In accordance with a third aspect of the invention, there is provided a zoom lens comprising, in order from an object side to an image side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, wherein a variation in magnification from a wide-angle end to a telephoto end is effected by moving the second lens unit toward the image side, shifting of an image plane due to the variation of magnification is compensated for by moving the fourth lens unit and focusing is effected by moving the fourth lens unit, a stop being disposed on the most object side of the third lens unit, the third lens unit comprising a positive lens (G3F) disposed on the image side of the stop and just behind the stop and having a convex lens surface facing the object side, at least one of the lens surfaces of the positive lens (G3F) being an aspheric surface, and a negative lens (G3R) of meniscus form disposed on the most image side and having a concave surface facing the image side, the curvature of which is stronger than that of an opposite surface thereof, the fourth lens unit consisting of a cemented lens composed of a positive lens, both lens surfaces of which are convex and at least one lens surface of which is an aspheric surface and a negative lens, and the following conditions being satisfied:
0.28 less than {|f2/fA|}xc3x97FNW less than 0.51
0.3 less than Rr/{(Nrxe2x88x921)fA} less than 0.6
where fA={square root over (fwxc2x7ft)}, and
where fw and ft are focal lengths of the zoom lens at the wide-angle end and the telephoto end, respectively, FNW is an F-number at the wide-angle end, Rr is the radius of curvature of the lens surface on the image side of the negative lens (G3R), Nr is the refractive index of material of the negative lens (G3R), and f2 is the focal length of the second lens unit.
In accordance with a fourth aspect of the invention, there is provided a zoom lens comprising, in oder from an object side to an image side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, wherein a variation in magnification from a wide-angle end to a telephoto end is effected by moving the second lens unit toward the image side, shifting of an image plane due to the variation of magnification is compensated for by moving the fourth lens unit and focusing is effected by moving the fourth lens unit, a stop being disposed on the most object side of the third lens unit, the third lens unit comprising a positive lens (G3F) disposed on the image side of the stop and just behind the stop and having a convex lens surface facing the object side, at least one of lens surfaces of the positive lens (G3F) being an aspheric surface, and a negative lens (G3R) of meniscus form disposed on the most image side and having a concave surface facing the image side, the curvature of which is stronger than that of an opposite surface thereof, the fourth lens unit consisting of a cemented lens composed of a positive lens, both lens surfaces of which are convex and at least one lens surface of which is an aspheric surface and a negative lens, and the following conditions being satisfied:
0.28 less than {|f2/fA|}xc3x97FNW less than 0.51
0.6 less than f3/f4 less than 1.15
where fA={square root over (fwxc2x7ft)}, and
where fw and ft are focal lengths of the zoom lens at the wide-angle end and the telephoto end, respectively, FNW is the F-number at the wide-angle end, and f2, f3 and f4 are focal lengths of the second lens unit, the third lens unit and the fourth lens unit, respectively.
In accordance with a fifth aspect of the invention, there is provided a zoom lens comprising, in order from an object side to an image side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, wherein a variation in magnification from a wide-angle end to a telephoto end is effected by increasing an interval between the first lens unit and the second lens unit and decreasing an interval between the second lens unit and the third lens unit, shifting of an image plane due to the variation of magnification is compensated for by moving the fourth lens unit and focusing is effected by moving the fourth lens unit, the third lens unit consisting of, in order from the object side to the image side, a stop, a cemented lens composed of a positive lens, both lens surfaces of which are convex and a negative lens, and a negative lens of meniscus form having a concave surface facing the image side, and the fourth lens unit consisting of a positive single lens having a convex surface facing the object side or a cemented lens composed of a positive lens and a negative lens.
In accordance with a sixth aspect of the invention, there is provided a zoom lens comprising, in order from an object side to an image side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, wherein a variation in magnification from a wide-angle end to a telephoto end is effected by moving the second lens unit toward the image side, shifting of an image plane due to the variation of magnification is compensated for by moving the fourth lens unit and focusing is effected by moving the fourth lens unit, the first lens unit comprising two or more positive lenses, the third lens unit having a stop disposed on the most object side thereof and comprising a positive lens having a convex surface facing the object side, the curvature of which is stronger than that of an opposite surface thereof, and a negative lens of meniscus form having a concave surface facing the image side, the curvature of which is stronger than that of an opposite surface thereof, the fourth lens unit consisting of a positive single lens having a convex surface facing the object side, the curvature of which is stronger than that of an opposite surface thereof, and the following condition being satisfied:
0.28 less than |f2/fA|xc3x97FNoW less than 0.52
where fA={square root over (fwxc2x7ft)}, and
where fw and ft are focal lengths of the zoom lens at the wide-angle end and the telephoto end, respectively, FNoW is the F-number at the wide-angle end, and f2 is a focal length of the second lens unit.
In accordance with a seventh aspect of the invention, there is provided a zoom lens comprising, in order from an object side to an image side, a first lens unit of positive refractive power, a second lens unit of negative refractive power a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, wherein a variation in magnification from a wide-angle end to a telephoto end is effected by moving the second lens unit toward the image side, shifting of an image plane due to the variation of magnification from a wide-angle end to a telephoto end is effected by focusing is effected by moving the fourth lens unit, the third lens unit having a stop disposed on the most object side thereof and comprising a positive lens having a convex surface facing the object side, the curvature of which is stronger than that of an opposite surface thereof, and a negative lens of meniscus form having a concave surface facing the image side, the curvature of which is stronger than that of an opposite surface thereof, the fourth lens unit consisting of a positive single lens having a convex surface facing the object side, the curvature of which is stronger than that of an opposite surface thereof, and the following conditions being satisfied:
xe2x80x830.28 less than |f2/fA|xc3x97FNoW less than 0.52
0.6 less than f3/f4 less than 1.25
where fA={square root over (fwxc2x7ft)}, and
where fw and ft are focal lengths of the zoom lens at the wide-angle end and the telephoto end, respectively, FNoW is the F-number at the wide-angle end, and f2, f3 and f4 are focal lengths of the second lens unit, the third lens unit and the fourth lens unit, respectively.
In accordance with an eight aspect of the invention, there is provided a zoom lens comprising, in order from an object side to an image side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, wherein a variation in magnification from a wide-angle end to a telephoto end is effected by moving the second lens unit toward the image side, shifting of an image plane due to the variation of magnification is compensated for by moving the fourth lens unit and focusing is effected by moving the fourth lens unit, the third lens unit having a stop disposed on the most object side thereof and comprising a positive lens unit having a stop surface facing the object side, the curvature of which is stronger than that of an opposite surface thereof, and a negative lens of meniscus form having a concave surface facing the image side, the curvature of which is stronger than that of an opposite surface thereof, the fourth lens unit consisting of a positive single lens having a convex surface facing the object side, the curvature of which is stronger than that of an opposite surface thereof, and the following condition being satisfied:
xe2x80x831.80 less than N2f less than 1.95
where N2f is the refractive index of material of a negative lens disposed on the most object side of the second lens unit.
Further, there is provided an optical apparatus comprising a zoom lens according to any one of the first to eighth aspects of the invention.
The above and further objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.